WCDMA has been chosen as the radio technology for the paired bands of the UMTS. Consequently, WCDMA is a common radio technology standard for third-generation wide-area mobile communications.
A fast and efficient random access procedure has been defined in the WCDMA system, wherein random access is based on a slotted Aloha transmission of a random access burst. The burst contains a preamble part, where a cell specific preamble code is used to transmit a preamble sequence randomly picked by the mobile station. The preamble sequence is detected in the receiver using a matched filter, and specifies a subset of a code tree that defines the channelization code (short code) used for an RACH (Random Access CHannel) message part.
In particular, a random access communication between a mobile station and the base transceiver station is established by means of an uplink PRACH (Physical Random Access Channel) and a downlink AICH (Acquisition Indication Channel), wherein the PRACH carries a RACH (Random Access Channel) through which the mobile station communicates with the base transceiver station. The mobile station transmits a random access message to the base transceiver station via the RACH when it wants to initiate a dedicated channel for e.g. a speech connection or it has some packet data which are to be transmitted. The base transceiver station transmits acknowledgments of RACH preamble signatures to the mobile station on the AICH, according to which the RACH message transmission is started.
However, in the common proposals for the WCDMA system, only a subset of all possible PRACH and AICH access slots is used, which limits the capacity of the RACH. This is not desirable especially in cases of high network loads.
Moreover, the number of access slots of the PRACH and the AICH is low. A preamble and an acquisition indicator (AI) may be transmitted every third or fourth access slot, so that the amount of hardware resources to be allocated to the RACH reception is low. If a single PRACH with such a limited capacity is not sufficient for a random access procedure, several PRACH and AICH channels per cell of the base transceiver station have to be configured. This leads to an undesirable complex random access procedure.